Photoconductive cell



April 1953 N. c. ANDERSON 2,636,100

PHOTOCONDUCTIVE CELL Filed March '7, 1951 IN vavron By N. C. ANDERSON ATTOR/Vfy Patented Apr. 21, 1953 to -FireycCorpration, Cambridge, Mass, acorporation ofMassachusetts Application-March 7, 1951, Serial No. 214,375

3 Claims.

This invention relates to improvements. in photoelectric cells of the photoconductive type.

An object of this invention is to simplify the grid structures heretofore used in wide-directional response photoconductive cells whereby such cells can be manufactured more easily and cheaply.

Another object is to increase the directional response of photoconductive cells.

Another object is to reduce the output impedance of particular photoconductive cells,

characterized by simple grid structures and wide directional response, to values suitable for coupling to the input of conventional amplifying apparatus.

Another object is to provide a relatively low output impedance photoconductive cell having a workable sensitivity.

Basically, a photoconductive cell, or a photoresistivecell asit is sometimes called, comprises a. material whose electrical impedance varies in response to the intensity and wave length of radiant energy impinging. thereupon. This material is, preferably, located within a hermetically sealed envelope and, is usually deposited on the inner walls of the envelope, or on the surface of a supporting structure positioned within the envelope. Electrical contact is made to this material by a plurality of conducting electrodes or the elements of a conductinggrid structure associated with the surface of the envelope walls or supporting structure, as the case may be. Conventional feed-through pins or terminals sealed;

in the walls of theenvelope provide the necessary external electrical connections to the electrodes o'rgrid structure and to the photoconductive material'.

In the prior art, the electrodes or grid structure and the contacting photoconductive material have assumed various geometrical forms depending, generally, upon the directional response, sensitivity and output impedance characteristics required of a particular cell.

Generally speaking, simple electrode arrangements have proven unsatisfactory for use with many photoconductive materials because of the excessively high output impedance of; the resulting cell. Consequently, it been customary to deposit or otherwise contact; the. photoconducti-ve material to the elementsof a fairly complex and structure to reduce the output impedance of the cells to values suitable for coupling to the inputs or conventional electrical apparatus. These complex, grid structures have consisted essentially of a, plurality of parallel, closely spaced,

2. conducting elements, sometimescalledlines. Alternate elements were-connected to one pinof the envelope, and the remaining elements were conneotedto the other pin of the envelope whereby the two sets of intercalated conducting elementsbroke up the photoconductive material into aplurality of parallel resistances. By a complex. grid design, sufficient incremental portions of a v photoconductive mass could-be connected in parallel to reduce the cell output impedance to a workable value The grid structures of photoconductive cells designed for applications requiring wide directional response were further complicated in the attainment of this requirement. Cells for fire detectors, for example, should be capable, of at least responding to radiations from fire which might emanate from directions corresponding to the radii of a hemisphere, with the cell being. located at the intersection point of said radii; otherwise, an increased number of cells would have to be used to supervise a given volume for fire.

This directional response has been. substantially attained; heretofore, by grid structures comprising numerous elements extending for degrees on the cylindrical, inner wall of a cell envelope and interconnecting a relatively large area of photoconductive material. The cells utilizing multi-element grid structures were relatively expensive to manufacture, however, because of the detailed procedures required to apply and interconnect the many grid elements.

Accordingly, a. preferred embodiment oi the novel, photoconductive cell of. this invention contemplates a simple grid arrangement applied directly to the cylindrical. shaped, inner wall of a cell envelope and contacting a small, cylindrical area of photoconductive material also applied to the inner envelopewall. This grid comprises, generally, two circular-ring elements of conducting material, such as aquadag or graphite, both of which elements. are in a closely spaced, parallel relationship with respect to one another. Thus, a short height, cylindrical portion ofenvelope wall appears therehetween. To this wall portion, and also on top of both grid elements, is deposited the thin, cylindrical layer of photoconductive. material whereby the conductance between the two grid elements is determined by the interconnecting cylinder of photoconductive material.

This arrangement provides for greater than a hemispherical response. because the photoconductive material is actively exposed in a 360 derelatively fine and are also,

gree cylindrical ring. Furthermore, inasmuch as the incremental areas of the photoconducting cylinder are effectively connected in parallel so that the impedance of each area is a function of the short height of the cylinder rather than the longer, circumferential distance of the cylinder, the output impedance of the cell is sufficiently low so that the cell may be coupled directly to conventional amplifying apparatus.

In order that all of the features of this invention and the mode of operation thereof may be readily understood, a detailed description follows hereinafter with particular reference being made to the drawings, wherein:

Fig. 1 is a perspective view of a preferred embodiment of the photoconductive cell of this invention;

Fig. 2 is a perspective view of the cell of Fig. 1 with the cell base removed and a portion of the envelope broken away; and

Fig. 3 is an enlarged developed showing of the grid structure shown in Figs. 2 and 3.

In the preferred embodiment shown in the drawings, the photoconductive cell comprises an envelope 4, preferably constructed of glass which will readily transmit the radiant energy to which the enclosed photoconductive material 13 is responsive. Two parallel and closely spaced grid elements 5 and 6 are applied directly to the inner wall of this envelope. Grid element 5 is cylindrical in shape and is continuous for a 350 degree peripheral path along the inner wall of envelope 5. Grid element 6, which is also cylindrical in shape, is discontinued for a small portion of its peripheral path so as to prevent contact with conductor 7. Conductor 1, which is applied directly to the inner wall of envelope 4, interconnects grid element 5 and feed-through pin 9. Conductor 8, which is also applied directly to the inner wall of envelope 4, interconnects grid element and feed-through pin ll], Consequently, external electrical connections can be made to both of the grid elements by contacting feedthrough pins 9 and i0.

Grid elements 5 and 6, and conductors l and 8, can be of graphite applied by a lead pencil, or the resulting solid deposited from an aqueous colloidal suspension, such as aquadag. It should be understood, however, that other suitable conducting materials which lend themselves to application in relatively fine lines can also be employed.

Pins 9 and Ill are preferably constructed of a conducting material having substantially the same linear coefiicient of expansion as the glass of envelope 4.

The permanence of the junction between the lower portions of conductors 1 and 8, and their respective pins 9 and It], can be improved by an application of silver paste to the envelope area immediately surrounding the sealed-in portions of the pins prior to the application of the conductor material to the glass envelope and the pins.

' Glass press H seals feed-through pins 9 and it to envelope 4, so that the envelope will be hermetically sealed by glass tip-oft I2.

Photoconductive material [3 is deposited upon and between grid elements 5 and 5, so that the electrical impedance between these elements is determined, for all practical purposes, by the impedance of the interconnecting cylinder of photoconductive material. Changes in this impedance value will occur whenever the appropriate frequencies of radiant energy impinge upon the ef 4 fective portions of the photoconductive cylinder.

The portions of grid element 6 immediately adjacent conductor 1 are, preferably, spaced therefrom by a distance of at least one and onehalf times the spacing between grid elements 5 and 6 so as to prevent partial shorting of the effective photoconductive cylinder between the grid elements 5 and 5. Likewise, conductors a and 8 are, preferably, located on opposite sides of envelope 4, or degrees from one anoth r as is represented by the developed showing of Fig. 3, to prevent similar partial shorting of the effective photoconductive cylinder. These partial shorting paths are undesirable because they reduce the sensitivity of the cell to radiations impinging upon the photoconductive cylinder between grid elements 5 and B.

In Fig. 1, envelope 4 and feed-through pins 9 and It are coupled to a double-contact bayonet type base 14, which will readily mate with a conventional bayonet type of socket. Other types of cell bases will house the cell envelope just as satisfactorily, however. Contacts l5 and it provide the necessary external connections to feedthrough pins 9 and ID.

The directional response of the photoconductive cell of this invention is limited by the relative size and shape of base It. For example, if a portion of base It is in the direct line of sight between a radiation source and the photoconductive material between elements 5 and '6, the response of the cell will be impaired. The cell is fully responsive, however, to radiations emanating from sources having an unobstructed line of sight to the photoconductive cylinder between elements 5 and 5. It is immaterial whether these radiations impinge upon the out side surface of this photoconductive material by passing through the portion of envelope t directly contacting the material, or by impinging upon the inside surface thereof by passing through the portion of glass envelope 5 near tipoff [2. The directional response of the cell is, therefore, somewhat less than a full sphere because of the obstruction offered by base is to particular radiations. It should be understood, however, that in many cases even though base Hi is within the aforementioned line of sight, the cell will, nonetheless, respond because of radiation reflections from ceilings or walls or other objects. ,7

Photoconductive material [3 can be of any of the types heretofore used in similar cells. It is preferable, however, that a material having a relatively low impedance value per unit area be used, such as lead sulfide, to kee the cell output impedance to low value. These materials can be applied to the envelope wall and activated by processes which are well known in the art.

Regardless of the photoconductive material used, the output impedance of the cell is rela tively low because of the short interconnecting photoconductive path between grid elements 5 and 6 which extends for substantially the complete inner circumference of envelope t. The new cell design, notwithstanding the relatively low output impedance characteristic thereof, also has a practical sensitivity value.

What is claimed is:

l. A photoconductive cell, comprising a hermetically sealed envelope, a cylindrical ring of photoconductive material positioned on the wall of said envelope, a first conducting element contacting the upper portions of said cylinder, a second conducting element contacting the bottom 0 Number said cylinder whereby said first and second conducting elements are interconnected by said photoconductive material, and means for hermetically sealing said cylindrical ring of photo- 5 conductive material.

NORMAN C. ANDERSON.

References Cited in the file of this patent UNITED STATES PATENTS Name Date 2,448,516 Cashman Sept. 7, 1948 

